1. Field of the Invention
The present invention relates to electrical heaters and, more particularly, to improved radiant heating modular units.
2. Description of Prior Art
Heretofore, individual heaters when butted together in a modular array to blanket the required material web width to the required temperature are incapable of maintaining a uniform temperature over every square inch of surface within tolerances of only a few degrees for the material to be acceptable. The basic construction and nature of the prior art heaters is such that the temperature falls off at the outer edges of the heating module. This is caused by (a) heating being lost through the metal case and (b) the geometry of the elements being such that, at best, the heating element is as far away from the metal casing as it is from an adjacent heating element. Consequently, when two heating modules are butted together in a multiple modular configuration, the unheated space between the heating elements of the adjacent heating modules is at least double that of the two adjacent heating elements in an individual heating module.
Accordingly, prior art heaters cannot be employed successfully with the heating elements arranged longitudinally in the machine direction for, as explained hereinabove, a heat striation pattern would be developed on the product passing underneath the area where two heaters are conventionally butted or interfaced. However, such prior art heaters can be used across the machine direction, but the useful width would be limited by the maximum length at which the heaters can be manufactured, based on the voltage, power requirements and watt density. Another drawback in the prior art heaters is that the heat pattern across the web cannot be "profiled" to compensate for other variables in the process, which variables are constantly affecting temperature uniformity. These usually are (a) temperature falling off at the extremities of the web because of conduction and natural convection; (b) variation in film thickness; (c) variation in web thickness; (d) a variation in the solvent or moisture content of the web; or (e) an unbalanced exhaust system.